106 BIRDS. 



is different. They must have a hard time of it, and 

 very many must perish by the way. As a sort of com- 

 promise between the two classes, we have the case, in 

 many ways unique among animal migrations, of Pallas's 

 sand-grouse, that remarkable little Asiatic wanderer, of 

 which five irruptions have found their way to these islands, 

 the last (1888-89) extending to their westernmost limits. 

 Intervals of from five to twelve years elapsed between 

 these invasions, whole generations in fact of sand-grouse 

 that never straggled to the Western continent. Here, 

 then, was clearly no case of transmitted instinct, but the 

 thousands that came so far from their native tundras 

 were evidently the children of circumstance, driven forth 

 by some sudden and unlooked-for alteration in the con- 

 ditions of life in those parts, some lack of food or maybe 

 some fall in temperature. The difficulty of drawing a 

 hard-and-fast line between the residents and the visitors 

 has already been indicated. It is only possible to lay 

 down certain principles, leaving room for numerous ex- 

 ceptions. Summer visitors are occasionally tempted to 

 stay the winter ; winter birds will bide with us in spring. 

 The divers and the fulmar and many others, residents 

 in the north of Scotland, are winter visitors only to the 

 coasts of England ; the whinchat breeds freely in the 

 north of Ireland, but is a winter visitor to the south. 

 And, before quitting the subject of migration, it is of 

 importance to mention the wanderings of many birds 

 within these islands. The robin is a case in point. Other 

 kinds, too, which in winter are found only on the sea-shore, 

 resort in the breeding season to inland moors and bogs. 

 Of such are the curlew and dunlin. 



Under normal conditions, the various groups of birds 

 affect a certain class of food. So much it is safe to say, 

 and the anatomist can, as a rule, make a 

 close guess from the form of the bill. Open- 

 air observation soon leads us, however, to supplement the 

 creed of the text-book, the ethics of the hard-billed and 



